Extraction columns or towers are devices wherein the mixing and the subsequent reseparation, generally in several subsequent passes, of two immiscible liquid and/or solid phases is realized in order to extract a component of one of the phases by means of the other one.
In an extraction column or tower, the component to be extracted is typically contained in water. The water is generally is heavier than the extracting phase, which is customarily made up of an organic solvent.
The countercurrent encounter between an aqueous phase containing the component to be extracted and an extracting phase, therefore, is realized by feeding the two phases respectively from the upper and the lower part of the extraction column or tower.
An extraction column or tower generally carries out the above mentioned mixing and subsequent separation in a repeated manner, in order to obtain an optimum separation of the desired component. This is realized through a structure with several operating sections, or stages. Each stage comprises a device for mixing the phases (for instance packings made up of material in pieces off various shapes, such as for instance Raschig rings, glass or ceramic saddle packings, perforated plates, or stirrers and the like). The extraction column or tower terminates at the top with a so called calm chamber, for the extracting phase after countercurrent extraction of the component to be extracted from the aqueous phase. The velocity in the calm chamber is such as to allow the reseparation of the phases.
The efficiency of an entire extraction column or tower is expressed by the concept of theoretical stages or "plates", which is equivalent to the number of discontinuous extractors in the entire column is expressed.
An extraction column or tower, be it with one or several stages, optimally performs both its quantitative and qualitative functions, when the diameter and height dimensional parameters have been set as near as possible to ideal ones that allow the most suitable remixing or diffusion for the treatment and an optimum subsequent reseparation in as short as time that as possible.
It is known that an efficient remixing or dispersion is achieved in a device having a diameter considerably smaller than the height, as in a large section two phases that are caused to encounter here, tend to produce channelization, and therefore travel reciprocally without splitting themselves into small masses. This splitting of the phases into small masses is necessary to achieve a good mass transfer, and therefore requires a high number of theoretical plates in the device. On the contrary, a fast and optimum reseparation is obtained in volumes wherein the measure of the surface is considerably higher than the height, as the larger cross section provides a greater ease of reseparation of the two phases due to the different surface tensions.